1. Field of the Invention
The present invention is related to dual hybrid mode dielectric resonator band pass filters. More particularly, the present invention concerns filter realizations in tubular enclosures which are suitable for use in broadcast receivers, phased array radar applications and other applications requiring large quantity microwave narrow band pass filters.
2. Discussion of Background
The use of low pass, high pass and band pass filters in microwave systems is well known and is used to achieve results similar to the use of such filters at low frequencies to separate frequency components of a complex wave.
Early attempts at providing waveguide type of filters involve the utilization of the lumped-circuit method of cascading several filter sections together which was copied in the sense that microwave filter sections were cascaded with the spacing between the sections being any odd number of quarter wavelengths. The theory being that the greater the number of cavities used, the flatter the pass band and the skirts of the pass band become steeper. As a practical matter, however, the insertion loss in the pass band increases with the number of resonators.
Recent developments with respect to dual-mode band pass filters as in the article entitled "Narrow Band Pass Waveguide Filters" by Atia and Williams, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-20, pages 258-265, April 1974 and "Dual-Mode Canonical Waveguide Filters" by Williams and Atia, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-25, pages 1021-1025, December 1977, and U.S. Pat. No. 3,969,692, July 13, 1976, "Generalized Waveguide Band Pass Filters", and U.S. Pat. No. 4,060,779, Nov. 29, 1977, "Canonical Dual Mode Filters" possess significant performance advantages over the above discussed conventional waveguide realizations which are detailed for example in "Microwave Filters, Impedance Matching Networks and Coupling Structures" by Matthaei, Young and Jones, New York: McGraw-Hill, 1965. These advantages of the dual mode band pass filters are especially significant in applications where the mass and the volume are critical. Other dramatic reductions in the filter size and the mass are achieved by using dielectric loading of the cavities with high-dielectric, low-loss temperature stable materials as reflected in the article by Fiedziusko entitled "Dual-Mode Dielectric Resonator Loaded Cavity Filters", IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-30, pages 1311-1316, September 1982.
Whether the filters are air-filled or dielectric-loaded dual-mode filters each of these type of structures in the prior art required that physically adjacent resonators be coupled to each other through iris slots or holes. These iris slots or holes required an extremely high degree of precision to provide the required accuracy for achievement of an exact filter response. Therefore, between each resonator, there was required a iris which had to be machined and silver plated which naturally led to major cost in producing to such extreme tolerances.
Therefore, in view of the high cost the utilization of these filters is restricted to applications where the performance, mass and size are extremely critical factors, as for example communication satellites. Normally their use was precluded in areas where cost is the major factor as where there are an extremely large number of filters to be used in, for example, phased arrays.
When filters such as hybrid dual mode dielectric resonators are configured, the most general band pass transfer function which is realizable utilizes a multiple coupled cavity structure which can be reduced to a canonical form containing the minimum number of coupling elements. FIG. 1 shows an equivalent circuit of a canonical form which consists of a number of identical resonant circuits 10 coupled in cascade by frequency invariant coupling elements M.sub.i, i+1, i=1, 2, . . . m having the same sign. Each resonant circuit 10 in one half is coupled to the corresponding circuit in the other half by means of a specified sign cross coupling element M.sub.i, n-i, i=1, 2, . . . m.
When the dielectric loaded resonators excited in hybrid mode (HEH.sub.11), are used in the canonical form, the result is shown in the FIG. 2. The Hybrid mode characteristics are discussed in Applicant's article entitled "New Results in Dielectric Loaded Resonators", IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-34, No. 7, July 1986, pages 815-824. The realization of FIG. 2 is similar to the realization of a circular waveguide form excited in TE.sub.111 modes described in the above-referred to article "Dual-Mode Canonical Waveguide Filters", 1977 and U.S. Pat. No. 4,060,779; (November, 1977). The cascade couplings of FIG. 1 are provided in FIG. 2 by the circular iris 20 separating each dielectric resonator 25. The coupling screws 27 are located at a 45.degree. angle to the direction of the degenerate dual modes and provide cross couplings. The relative signs of any two cross couplings are determined by the relative directions of the corresponding coupling screws with the same sign being dictated by parallel screws and opposite signs being dictated by perpendicular screws. Although not shown, it is a feature of the dual mode structure, whether air-filled or dielectric, that there are two tuning screws associated with each resonator in order to adjust the resonant frequency of each set of orthogonal modes. This is discussed in the above-discussed April 1974 and December 1977 articles by Atia and Williams.
The realization of cascade couplings produced by the iris separation of the resonators in FIG. 2 presents the above-discussed difficulties concerning the manufacture of these iris elements and the extreme accuracy with which they must be manufactured. Thus, although dual mode dielectric resonator filters, which are extremely light and extremely space conservative, are available, from a practical standpoint the cost to manufacture prohibits their use in most large quantity microwave narrow band pass filter applications.